The invention relates to a biodegradable fixation element, in particular for anchoring bone implants, having a body with anchoring elements arranged on its outer side.
A biodegradable fixation element in the form of an interference screw is disclosed in OP Journal 14 (1998) pages 278-284 “Biodegradierbare Interferenzschrauben in der Kreuzbandchirurgie” A. Weiler et al., Georg Thieme Verlag Stuttgart, N.Y.
Biodegradable materials are ones which are completely absorbed by the body. They also include composite materials which are only partially taken up by the body.
Interference screws have the purpose of anchoring a tendon or ligament transplant or implant in a bone. A channel is drilled into the bone in which the transplant is inserted. The interference screw is screwed into the intermediate space between the transplant and the inner wall of the channel, so that the transplant is then clamped between the screw and the inner channel wall.
Considerable forces arise in such a transplant, for example in the cruciate ligaments in a knee joint, so that the clamping force must be correspondingly large to ensure a secure anchoring. The screw is therefore provided with anchoring elements in the form an outer threading, which penetrates into the bone material at the inside of the channel. At the same time, the outer threading engages the transplant to be anchored. To exert the considerable mechanical clamping force, the fixation element must have a certain minimal geometric size.
When the fixation element begins to degrade biologically, a correspondingly large hollow space arises. Bone material can grow into this space with time, so that the transplant remains firmly anchored. If the biological degradation does not take place parallel with the bone growth, instable conditions can arise in that the bone growth is not sufficient or is not sufficiently fixed, while hollow spaces have already arisen due degradation.
International patent application WO 99/44544 discloses a surgical implant for fixing a bone block into a drillhole in a bone. The implant has an elongated hollow body and is manufactured from a bioabsorbable polymer. At least one gripping element is provided on the elongated body for locking the implant into the drillhole. The body is provided with at least one slot extending over the overall axial length of the body. The slotted body can be squeezed into the space between the drillhole and the bone block for fixing said bone block within said drillhole. The body may be provided with pores for allowing a growth of bone material into said body.
The basic principle is to fix the rigid bone block by squeezing the slotted body between the block and the drillhole. If one has to anchor transplants of flexible material, for example a tendon, the fixation element needs to be rigid and not deformable or squeezable.
Such fixation elements are subject of the present application.
If revisions are required and a new transplant is to be applied, corresponding borings must be made for inserting the fixation element. However, if sufficient bone material is not present, these borings cannot be properly made and correspondingly the new transplant cannot be fixed.
An object of the present invention is to provide a biodegradable fixation element of the mentioned type, which allows the formation of a bone network structure in the region of the transplant.
It is a further object of the transplant of the present invention to maintain the anchoring of the transplant and the mechanical strength of said body during biodegradation as long as possible but allowing a growth of bone material into said interior of said body.